'\" t
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH mq_receive 3 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
mq_receive, mq_timedreceive \- receive a message from a message queue
.SH LIBRARY
Real-time library
.RI ( librt ", " \-lrt )
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.P
.BI "ssize_t mq_receive(mqd_t " mqdes ", char " msg_ptr [. msg_len ],
.BI "                   size_t " msg_len ", unsigned int *" msg_prio );
.P
.B #include <time.h>
.B #include <mqueue.h>
.P
.BI "ssize_t mq_timedreceive(mqd_t " mqdes ", \
char *restrict " msg_ptr [. msg_len ],
.BI "                   size_t " msg_len ", unsigned int *restrict " msg_prio ,
.BI "                   const struct timespec *restrict " abs_timeout );
.fi
.P
.ad l
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR mq_timedreceive ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
.BR mq_receive ()
removes the oldest message with the highest priority from
the message queue referred to by the message queue descriptor
.IR mqdes ,
and places it in the buffer pointed to by
.IR msg_ptr .
The
.I msg_len
argument specifies the size of the buffer pointed to by
.IR msg_ptr ;
this must be greater than or equal to the
.I mq_msgsize
attribute of the queue (see
.BR mq_getattr (3)).
If
.I msg_prio
is not NULL, then the buffer to which it points is used
to return the priority associated with the received message.
.P
If the queue is empty, then, by default,
.BR mq_receive ()
blocks until a message becomes available,
or the call is interrupted by a signal handler.
If the
.B O_NONBLOCK
flag is enabled for the message queue description,
then the call instead fails immediately with the error
.BR EAGAIN .
.P
.BR mq_timedreceive ()
behaves just like
.BR mq_receive (),
except that if the queue is empty and the
.B O_NONBLOCK
flag is not enabled for the message queue description, then
.I abs_timeout
points to a structure which specifies how long the call will block.
This value is an absolute timeout in seconds and nanoseconds
since the Epoch, 1970-01-01 00:00:00 +0000 (UTC),
specified in a
.BR timespec (3)
structure.
.P
If no message is available,
and the timeout has already expired by the time of the call,
.BR mq_timedreceive ()
returns immediately.
.SH RETURN VALUE
On success,
.BR mq_receive ()
and
.BR mq_timedreceive ()
return the number of bytes in the received message;
on error, \-1 is returned, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
The queue was empty, and the
.B O_NONBLOCK
flag was set for the message queue description referred to by
.IR mqdes .
.TP
.B EBADF
The descriptor specified in
.I mqdes
was invalid or not opened for reading.
.TP
.B EINTR
The call was interrupted by a signal handler; see
.BR signal (7).
.TP
.B EINVAL
The call would have blocked, and
.I abs_timeout
was invalid, either because
.I tv_sec
was less than zero, or because
.I tv_nsec
was less than zero or greater than 1000 million.
.TP
.B EMSGSIZE
.I msg_len
was less than the
.I mq_msgsize
attribute of the message queue.
.TP
.B ETIMEDOUT
The call timed out before a message could be transferred.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR mq_receive (),
.BR mq_timedreceive ()
T}	Thread safety	MT-Safe
.TE
.SH VERSIONS
On Linux,
.BR mq_timedreceive ()
is a system call, and
.BR mq_receive ()
is a library function layered on top of that system call.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH SEE ALSO
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_notify (3),
.BR mq_open (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR timespec (3),
.BR mq_overview (7),
.BR time (7)
